Scores for non player entities are cleared as them pass through portals, no matter thay are Nether Portals or End Portals.
To reproduce it:
Prepare a Nether Portal or a End Portal, an objective of any kind and a non player entity (for example, a cow).
Set a score for the entity and make it pass through the portal.
Use a command such as /scoreboard players get or something else to get the score.
You will find that none is set for the objective